Also deproceduring and uniting, but not widening, rowing or voiding.
The reason is, of course, to prevent ambiguity -- you are not allowed to
overload operators with operands varying [eg] only in the reference levels,
but you are with operands varying [eg] only in row levels, so you can
disambiguate operands with the wrong reference levels, but not with the
wrong row levels.

I suspect that at least rowing could have been moved into the other
camp [though there may be some subtleties involving the interaction between
rows and references], but pragmatically one may well want to overload, in
particular, "*" to mean different things for scalars, vectors and matrices.